Malaria is a parasitic disease caused by infection by species of the genus Plasmodium:

Example of Clinical features UK cases 2011

Plasmodium falciparumResponsible for severe disease and malaria-related deaths.Incubation 7-14 days (up to 1 year if semi-immune); most travellers present within 8 weeks. Classical tertian and subtertian periodicity(paroxysms at 48- and 36-hour intervals) are rare;daily (quotidian) or irregular are more common. (1,149 cases in UK)

Plasmodium vivaxCauses benign tertian malaria - fever every third day. Incubation period of 12-17 days. Relapse due to dormant parasites in the liver. (416 cases in UK)

Plasmodium malariae Causes benign quartan malaria - fever every 4th day,but this is frequently not observed, particularly in early infection.Long incubation period (18-40 days). Parasites can remain dormant in the blood. 5-10% present over a year after infection. With chronic infection, can cause nephrotic syndrome. (31 cases in UK)

Plasmodium knowlesi, has recently emerged. It is distributed across Southeast Asia and is often misdiagnosed by microscopy as P. malariae. However, it is potentially more serious, causing severe malaria with a rate of 6-9% and with a case fatality rate of 3%.

"The malaria parasite requires specific human and mosquito tissues to complete its life cycle. Once inside a human, the parasite develops and multiplies, causing periodic bouts of flu-like symptoms, including fever, headache, and chills. The developing parasites destroy red blood cells, which may cause death by severe anemia as well as by the clogging of capillaries that supply the brain or other vital organs with blood. The deadliest of the four species of the parasite is Plasmodium falciparum, a species most likely to be transmitted by the mosquito Anopheles gambiae."